1. Field of the Invention
The present invention generally relates to electron spectroscopy and, more particularly, to improvements in X-ray photoelectron spectroscopy.
2. Description of the Prior Art
Electron spectroscopy for chemical analysis (ESCA) has become a useful technique to study surface phenomena. Basically, in an ESCA spectrometer, kinetic energies of electrons, which were ejected from the surface of a sample, are measured. Based on those measurements it is possible to determine what atoms are present at the sample surface and their relative abundance. Also, by observing small shifts in the energies of the emitted electrons, compared to their total energies, one can derive information regarding the chemical environment of the atoms, i.e., what their neighboring atoms are and how they are bonded to these neighboring atoms.
One ESCA spectrometer, which is available commercially from Hewlett-Packard Co. of Palo Alto, California is an X-ray ESCA spectrometer. In it, photons from an X-ray source are directed and bombard the sample surface. Due to the photon energy which is absorbed by the sample surface, photoelectrons hereinafter simply referred to as electrons, are ejected from the sample surface. These electrons are passed through an analyzer and therefrom to a detector. The Hewlett-Packard (HP) X-ray ESCA spectrometer is well known by those familiar with the art. This model is described in the "Hewlett-Packard Journal", July 1973, which is published by the manufacturer.
In the photoelectron spectrometer, since the measurements are made of the kinetic energies of the electrons as they leave the sample surface, in order to properly interpret the measurements or data, it is necessary to know the vacuum level of the sample, i.e., the sample work function and its surface potential, with respect to some reference, such as system common. Assuming that the sample's work function is constant, the sample's surface potential need be known.
When studying the surface phenomena of a good conductor, such as a metal or semiconductor for all practical purposes the surface potential of the sample is the same as the sample's bulk potential. Thus, by connecting the back side of the sample bulk to the system common the surface potential is actually the same as that of the system common, i.e., is known. Therefore, the measurements of the energies of the ejected electrons can be interpreted properly. However, when studying the surface chemistry of an insulator, by connecting the insulator back side to the system common the insulator's surface potential is not known, since in an insulator its surface potential may differ significantly from the insulator bulk potential.
The problems, presented by the surface potential of an insulator, in studying the surface chemistry of insulators have been appreciated in the prior art. In the "Hewlett-Packard Journal" of July 1973, the use of a flood gun is described. The flood gun is intended to supply low-energy electrons to the insulator surface and thereby reduce the positive surface potential which is created when the surface is struck by the X-ray photons, which cause the electrons to be ejected.
Although the use of the flood gun as described in the prior art may provide some advantages, it is not satisfactory when precise measurements are required, including the need for observations of small energy shifts. With the flood gun it is not possible to determine the actual insulator's surface potential or relate it to a known potential. Thus, all measurements cannot be made as precisely as desirable. Furthermore, small shifts in electron energies cannot be interpreted, to provide accurate information relating to atoms neighboring those from which the electrons are ejected. Other disadvantages of the use of the floor gun as proposed in the prior art will be discussed hereinafter.